1. Field of the Invention
The present invention relates to a focus detection apparatus, and more particularly, to a focus detection apparatus for an object.
2. Description of the Related Art
Conventionally, various proposals have been made for focus detection technologies in image pickup apparatus such as cameras and video cameras. For example, there is proposed a phase difference method using so-called TTL Through the Lens), in which a separation unit is provided in an optical path in an imaging optical system and a focus state is detected using separated beams.
In addition, there is also proposed an external measurement method using so-called non-TTL, in which a focus state is detected using a beam of outside light that is different from the beam in the imaging optical system. Further, there is proposed a method in which a focus state is detected using an image signal output from an image pickup element.
In the focus detection apparatus of the phase difference method and the external measurement method among the above-mentioned methods, photoelectric conversion of a beam from the object is performed by a photoelectric transducer in the focus detection apparatus, and the obtained electric charges are accumulated and read out as image signals. Further, the image signals read out from the photoelectric transducer are used for performing correlation computing to determine a deviation amount of the images, namely a phase difference. In this case, a coincidence degree between two images is used as a correlation score for determining a target value until an in-focus point. In general, a phase difference for which the correlation score becomes extremal and largest is set as the target value with high reliability. Then, the target value is converted into a target position of a focus lens from a defocus amount from the in-focus point or distance information to the object, to thereby perform drive control of the focus lens.
By the way, there are proposed various methods for controlling the charge accumulation operation in focus detection sensors of the phase difference method and the external measurement method. For instance, there is known a control method by automatic gain control (AGC), in which the accumulation operation is terminated when a predetermined signal level is reached. As another example, there is known a control method in which the accumulation operation is terminated at a time point when a predetermined maximum accumulation time has elapsed even if the predetermined signal level has not been reached. In addition, both the above-mentioned two types of accumulation control methods are sometimes used so as to cover a wide dynamic range of object luminance under various image taking conditions.
As a sensor for focus detection in the phase difference method or the external measurement method, a linear sensor formed in a single line or an area sensor formed of a plurality of line sensors is used. In some area sensors, each area thereof is connected to an AGC circuit and accumulation control is performed so that each area can output a signal with an optimal contrast independently. In this way, by disposing a plurality of areas in an image taking screen for performing focus detection, it is possible to perform focus detection computing with an optimal contrast for the focus detection area.
However, if the sensor is divided into a plurality of areas an image may be formed over a plurality of areas. For instance, in the phase difference method, when a large defocus is generated, two formed images may not be contained in one area. In addition, in the external measurement method, depending on a distance of the object, two formed images of an object may be apart too much to be contained in one area so that the images are formed over areas.
By setting an appropriate AGC for each area and performing the correlation computing after linking pieces of data between areas, it is possible to perform focus detection with the use of a plurality of areas. However, a plurality of accumulation operations is necessary, and hence process time of the focus detection is increased. Particularly in a low illuminance environment, because the accumulation time is long, it is difficult to deal a real time process in taking a moving image or the like. Therefore, as a conventional example, there is proposed a method for performing the focus detection computing using two images over a plurality of areas.
For instance, Japanese Patent Application Laid-Open No. 2006-184320 proposes the following method. If focus detection cannot be performed when correlation computing is performed in an m-th area, data of an (m−1)th area or an (m+1)th area as a neighboring area is converted into an arbitrary accumulation time and is linked to data of the m-th area. Then, a correlation computing range is enlarged until the focus detection can be performed. With this configuration, it is possible to cover the case where an image is formed in different areas when a large defocus is generated.
In addition, Japanese Patent Application Laid-Open No. 2008-009279 proposes a method of switching between a first mode in which the accumulation control is performed independently for each area for independently reading out a signal output and a second mode in which the accumulation control is performed integrally for all areas for reading out a signal output. In the configuration of Japanese Patent Application Laid-Open No. 2008-009279, in the second mode, a plurality of areas are linked for performing the accumulation control similarly to a single linear sensor, and individual operating circuits are sequentially driven to output the signals. Therefore, without disposing a dedicated sensor for large defocus, it is possible to perform focus detection over a plurality of areas.
However, in the conventional methods disclosed in Japanese Patent Application Laid-Open Nos. 2006-184320 and 2008-009279, a time lag in the accumulation control of different areas or a difference of accumulation sensitivity between areas is not taken into consideration. Therefore, in the strict sense, the obtained data is different between areas. Therefore, according to the conventional methods of linking pieces of data between areas, different results are obtained between a case where the correlation computing of the same object is performed in a single area and a case where the correlation computing is performed over a boundary between areas, and hence an exact correlation computing result cannot be obtained in some cases.
In particular, in a high illuminance environment, a level difference between areas is conspicuously affected. There is a case where the correlation computing result is different from an inherent value depending on an illuminance condition. As a result, when an automatic focus operation is performed, the focusing operation may cause a blurred state.
In the phase difference method, two images are generally formed in one area as the focus state is closer to the in-focus state. Therefore, accuracy of the correlation computing between areas is improved more as the focus state is closer to the in-focus state. Therefore, it is possible to perform focus control always by feedback control. However, in the case of the external measurement method, which indicates data of distance to an object, a constant result is obtained regardless of the in-focus state or the non-focused state. In other words, the external measurement method is an open-loop focus control method. Therefore, in the case of the external measurement method, an influence of an error of the correlation computing result particularly at a boundary between areas is apt to increase. In addition, also in the case of the phase difference method, if a peak of a luminance level of the object exists at the boundary between areas at a time point when the in-focus state is realized finally, unlike inherent accumulation data, accuracy of the phase difference to be calculated by the correlation computing is decreased.